Tuning of high-frequency circuits



Dec. 17, 1929. w. L. CARLSON 1,740,331

TUNING OF HIGH FREQUENCY CIRCUITS Filed Oct. 4, 1928 FREQUENCYDIFFERENCE 1N KC.

D o B 600 \HPUT FREQUENCY m KC- I500 Inventor:

45 Wendell Lhcorr lsom,

y Wwu His Attorney.

Patented Dec. 17, 1929 UNITED STATES PATENT OFFICE WENDELL L. CARLSOLLQFBCHENECTADY, NEW YORK, ASSIGNOR T0 GENERAL ELECTRIC COMPANY, ACORPORATION 0]? NEW YORK TUNING- OF HIGH-FREQUENCY CIRCUITS Applicationfiled. October a, 1928. Serial No. 310,298.

ferent signal or broadcast channels simultaneously at a constantfrequency difference, or in obtaining a constant frequency output from areceived signal and a local oscillator in a superheterodyne receiver forexample.

As is well understood in the art, unicontrol or single control tuningrefers to the simultaneous operation of several tuning devices, usuallyeach in a separate circuit, by the same operating means, such as anindicating dial or knob, connected therewith by a common shaft or othersuitableoperating connection.

In accordance with present practice, tuning of high frequency circuitsis accomplished by variable capacities or condensers in connection withfixed inductances having fixed distributed capacities. The unicontroltuning of a plurality of circuits to the same frequency is thuscomparatively easy since it involves merely a duplication of the tuningmeans and other constants in each circuit and operating the tuning meanssimultaneously through the same tuning and movement range.

The problem of providing a constant difference in frequency responsebetween two unicontrolled circuits over a relatively wide tuning rangeis more difficult of solution and as ordinarily carried out involves notonly duplicate tuning means but equal changes in frequency for equalangular or linear movements of the tuning means, known as a straightline frequency tuning characteristic. This involves a fixed angular orlinear displacement of the moving or fixed parts of the tuning means ofthe two circuits between which a constant frequency difference isdesired. Thus heretofore it has been considered necessary to employtuning condensers pf the usual rotary plate type having the platesshaped to give a straight line frequenc tunmg characteristic, With anangular egree shift between rotors on the same shaft to make thecircuits connected therewith hold the desired constant frequencydiflerence over a relatively wide tuning range or band of frequencies.

Straight line frequency condensers of the usual rotary plate type arerelatively costly, being confined to an accurate plate shape, and occupya larger space than other more easily manufactured types, such asstraight line capacity or straight line wave length types. Furthermore,the angular. shift between the rotors' for unicontrol operation not onlyrequires an angular adjustment means between the condensers, but limitsthe normal 180 degree movement of each condenser to some lesser value inwhich to cover the full tuning range.

In a superheterodyne for example, in order to cover the broadcast bandand make one circuit follow the other at a wide frequency separation,such as 200 k. c. for example, the rotors of straight line frequencycondensers would be locked together out of step or synchronism at asubstantially 30 degree angular displacement, with identical inductancesand distributed capacities in each circuit. lhe total allowable rotationis thereby reduced from 180 degrees to 150 degrees and it is necessaryto cover the broadcast range in a 150 degree movement of the tuningmeans instead of a full 180 degree movement. Thus in addition todisadvantages in size and manufacturing cost, straight line frequencycondensers utilized in the unicontrol tuning of tWo circuits having arelatively wide frequency difference, have the added disadvantage of alimited movement range for a given'frequency band to be covered.

In a unicontrol tuning means, it is desirable for manufacturing reasonsto provide a unicontrol condenser unit comprising a plurality ofvariable condensers with the rotors integral with the shaft and withoutangular displacement, that is, with the rotors in step or synchronism,and for reasons of compactness therotors are preferably not of thestraight line frequency type. With this unit type of tuning means, allthe rotors are electricall connected with the shaft and frame and a sooften with a common bus wire in the set such as the ground bus, althoughthe condensers may be-insulated from each other if desirable ornecessary for isolating the circuits connected therewith.

The principal object of the present invention is to provide an improvedunicontrol circuit arrangement which permits the use of a simple, easilymanufactured variable condenser unit or tuning means of the abovedescribed type, in tuning two or more high frequency circuits, at leastone of which it is desired to operate at a constant frequency differencewith respect to the others.

Although the circuit arrangement and tuning means of the presentinvention are not limited thereto, they are partlcularly adapted for usein a superheterodyne receiver, and a further object of the invention'isto provide a circuit of that type which permits the use of a commongrounded rotor condenser unit of the above type or its equivalent in aunicontrolled superheterodyne.

The invention will now be described more fully in connection with theaccompanying drawing and its scope will be pointed out in the appendedclaims. In the drawing, Fig. 1 is an elementary diagram of twounicontrolled tuning circuits; Figs. 2 to 5 are similar diagrams of oneof said circuits with various values of inductance and capacitysubstituted therein; Fig. 6 is a curve diagram showing the difference infrequency response between the two unicontrolled circuits lllustrated byFigs. 1 to 5; Fig. 7 is a circuit diagram. of the tuned circuits ofunicontrolled recei'ver embodying the invention, and Fig. 8 is a similarcircuit diagram showing a modification.

Referring to Fig. 1, X is a tuned radio frequency circuit which may beconsidered as representing one or more such circuits with which asimilar circuit A is tuned by unicon trol means at a certain frequencydifference which is desired to be substantially constant. The circuitsshown comprise two variable capacities or condensers 10 and 11 whichhave a the same plate shape or tuning characteristic and movement range,have the same capacity range within the limits of their movements andare coupled for unicontrol or simultaneous operation as indicated bydotted connection 12. In a multituned circuit arrangement, suchcondensers are preferably on the same shaft in a gang unit. Connected inshunt with condensers 10 and 11 respectively to complete the circuitsare two inductances 13 and 14, together with the distributed capacitiesof the inductances and connecting leads thereto indicated by 15 and 16.

With one of the circuits, such as circuit X, designed to cover thefrequency band desired with movement of the condenser 10 between itsmaximum and minimum capacity positions, circuit A may be designed toprovide the desired difference in frequency response by windinginductance or coil 1A to a higher or lower value than inductance or coil13. In the present example, inductance 14 may be considered to be woundto a lower value .to which circuit X is tuned along abscissa 17 andfrequency difference between the two circuits along ordinate 18. In thiscurve, a frequency diiference of 200 k. c. is assumed by way of exampleand inductance 14 is designed to provlde the diiference in frequency atthe lower end of the tuning range in k. c. which is assumed for purposesof illustration to be approximately the broadcast band, from 600 k.'c.to 1500 k. c.

It will be seen that while the desired frequency separation is obtainedat 600 k. c., the separation increases to a wider value at 1500 k. c.,thus making a circuit of this character unsuitable for use in a receiverrequiring a constant frequency difference between certain tunedcircuits, such as asuperheterodyne receiver for example, in whichcircuit X would preferably be utilized for the radio frequency input tothe heterodyne and circuit A for the oscillator.

Modifications B, D, E and F of circuit A, with no changes in circuit X,are shown in Figs. 2 to 5 inclusive in each of which circuit X has beenomitted and in which the same reference numerals are used for the sameparts.

Referring to Figs. 2 and 6, the eifect of adding a series condenser 19in the circuit with other constants indentical with those in circuit Xof Fig. 1 is shown. The series capacity is adjusted to'provide thedesired frequency difference at one point, such as the lower end of thetuning range asbefore. It will be seen b referring to curve B that thefrequency di erence is noW reduced, in moving the tuning means throughthe range to the maximum frequency, thus making this circuit arrangementalso unsuitable for the purposes intended; As indicated in the figurebythe size of the units, the added series capacity is greater than themaximum capacity of condenser 11.

Curve D in Fig. 6 shows the eifect of an added shunt capacity 20, asindicated by Fig. 3, in the circuit with the same tuning condenser 11and distributed capacity 16. The' inductance and shunt condenser valuesin this case can be designed to bring the frequency separation to thedesired substantiall constant value at each end of the range. If; willbe seen that this arran ement has a some what opposite effect from t atof Fig. 2 using the series condenser, and that while it is possible-t0obtain the desired result at the. ends of the tuning range, thefrequency separation varies widel therebetween, In this circuit, theadded s iunt capacity is less than the maximum of condenser 11, butlarger than the distributed capacity 16, while the inductance 14 isslightly less than that in circuit A, of Fig. 1. j

-From aconsideration of the curves resulting from the circuits of Figs.1, 2 and3'and with the idea of keeping condenser 11 thesame, anacceptable and almost ideal curve E is produced by the design of thecircuit of Fig. 4,]in which only a slight added shunt capacity 21 and arelatively large series capacity 22-23-2t are used, together with acorresponding change in the design of inductance 14.

In this circuit inductance 14 is lessthan the corresponding inductanceof tuned circuit X of Fig. 1, and reater than inductance 1a in circuitsA and% of Figs. 1 and .3 respectively, While the variable tunincapacities in each circuit are the same as is disirable. The

, added shunt capacit is relatively small and may consist of a slig tincrease of distributed capacity or a small fixed capacity. The addedseries capacity is substantially three times the value of the maximum oftuning condenser 11. The series capacity is shown in three sections22-2324 to illustrate its relative size.

Substantially the same curve as E, may be obtained by changing theconstants of the circuit. 'One design lnvolving the use of a largerinductance for coil 14 than coil 13in Fig. 1,'is shown in Fig. 5. Withalarger inductance the total capacity, represented by the series andtuning condensers and distributed capacity, are reduced proportionatelyto maintain the tuning range and fre quencydifierence as before. Thus inthe circuit ofjFig. 5, the series capacity represented by twd'units 25and 26 is substantially twothirds ;of the total value employed in thecircuit of Fig. 4, and the tuning capacity 11 is likewise reducedsubstantially one-third as is also the distributed capacity 27.

The *change in the capacity of the tuning condenser does not complicatethe construction of a gang or multiple condenser unit of the unicontrol,common shaft type, since it involves no change in plate shape but onlythe omission of a certain number of rotor or stator plates, or both,when the condenser is assembled. i 1

The tuning variation in k. 0. between the higher and lower frequencycircuits should be substantially constant and is preferably less than 5k. c. from a certain constant value, such as 200 k. c. in the presentexample, for use in a superheterodyne type of receiver, in order thatthe selectivity of the receiver may be maintained, as is wellunderstood. The curve shown at E in Fig. 6 and the cor-' respondingcircuits in Figs. 4 and 5 are such that the intermediate frequency orfrequency.

These three tuned circuits, which may be i considered to represent aplurality of tuned circuits for any receiver or high frequencyelectrical apparatus, are tuned by a unit condenser, having threesections 28, 29 and 30 arranged for unicontrol operation as indicated bythe dotted connection 31. These condensers have plates of the same shapewhereby they have the same tuning characteristic and are arranged instep or synchronism, that is, with no angular displacement, so that thecapacity change for a given movement of the tuning means is the samewith respect to the maximum for each condenser in each circuit. Thecondensersmay be and preferably are all of the same capacity range aspermitted by the limits of their unicontrolled movement.

In the present example, condensers 28 and 30 are arranged to control tworadio-frequency circuits comprising the secondaries 32 of radiofrequency transformers 33 which are tuned thereby to substantially thesame frequency, whilecondenser 29 is arranged to control at a certainhigher frequency and at substantially a constant frequency difference,

an oscillator circuit comprising an inductance or oscillator grid coil34, a series capacity 35-36, the latter portion of which is adjustable,and an adjustable shunt capacity 37 for adjusting the overalldistributed capacity of the circuit. It will be noted that .theoscillator circuit follows the arrangement tuning capacity, although itis in efiect in shunt with the distributed capacity of the coil becauseof the relatively large size of the series capacity 3536 with'respect toit. This arrangement of total shunt capacity across the inductance,having the variable tuning and adjustable distributed capacity inparallel, and series connected with a fixed and variable capacity 3536across the inductance, permits of a rapid and accurate factoryadjustment when aligning the tuned circuits to the same frequency, anadjustment of the distributed capacity 37 being most effective inbringing the frequency difference to the desired value at the highfrequency end of the tuning range and an adjustment of theseriescapacity 36 being most effective for bringing the frequency differenceto the dethe other may be connected with another tube a sired value atthe low frequency end of the tuning range. These capacities are thenfixed and require no further adjustment.

In the present example one radio frequency circuit is connected with thedetector or frequency changer tube, indicated at 38, while such as radiofrequency amplification. Tube 38 is of the heater type having a heater38 for the oath ode 38 The detector outputs to the remainder of the setthrough the usual intermediate frequency transformer indicated at 39 andis coupled to the oscillator by an inductanceor coil 40 in the cathodelead 41. This coupling arrangement permits the use of a common shaft orgrounded rotor type of condenser unit, that is, a common bus connectionfor all units as indicated by connection of each unit with a commoncircuit wire 42. This connection is not possible with the usualoscillator-frequency changer circuit except by placing the coupling coilin the frequency changer grid lead, and is particularly adapted for usewith a heater type, separate cathode tube as indicated in the drawing,since the cathodecircuit then carries no heat- (not shown) for otherpurposes ing current and may be returned to any de-.

sired part of the circuit such as to the common bus 42 to which thecondenser units are connected. I

The oscillator tube maybe of the same type, as indicated, and as anoscillator is provided with the usual plate coil 43 inductively coupledwith grid coil 34:. Both tubes may have a suitable common cathodeheating current supply source 44 and anode voltage supply source 45represented by simple batteries, although any suitable source of energyforthe cathode and anode circuits may be used.

Referring now to Fig. 8, a modification of the circuit of Fig. 7 isshown, in which the oscillator and frequency changer functions arecombined by connecting a single tube e6 with both the radio frequencyand oscillator tuning circuits, the elements of which bear the samereference numerals as in the preceding figure, circuit 30 -32 beingomitted.

In this arrangement the tube is a three element type having its gridconnected with both tuned circuits through coupling condensers 47 and4:8, and having its plate circuit connected with output transformer 39and oscillator plate coil 43. This arrangement permitsthe grid of thetube to be controlled through condenser 47 in response to the signalfrequency tuned in by radio frequency circuit 28-82, and to respond tothe oscillation frequency in the adjusted oscillator circuit 2935-34=,which is unicontrolled withthe radio frequency circuit throughconnection 31.

This circuit arrangement does not differ in practical operation fromthat of Fig. 7 and is particularly adapted for use in a portablereceiver in which a reduction in the number of tubes is of primaryimportance. In this circuit, the coupling condenser 47 is made larger incapacity than coupling condenser 48, if it is desired to provide a lowvoltage drop to the grid while condenser 48 is of such capacity that thegrid connection.

forms no appreciable load on the oscillatory circuit. Both capacitiesare preferably'each greater than the grid to cathode capacity of thetube.

In the circuits shown in Figs. 7 and 8, the oscillator circuit ispreferably the one having the series and shunt capacities, for thereason that with a plurality of tuned radio frequency circuits allxceptthe oscillator circuit may then be identical and comprise merelyasimple inductance and capacity, although of course the oscillatorcircuit may comprise only such simple inductance and capacity, but thiswould necessitate providing the series and shunt capacities in each ofthe tuned radio frequency circuits in order to maintain the samefrequency difference. Thus in any case it is preferable to have thefrequency difference set up by the use of the series and shuntcapacities in a minimum number of the circuits, which in the presentexample is the i one circuit for the oscillator.

Thus it will be seen that by providing the circuit arangement of thepresent invention, a simple unicontrol tuning condenser means or unitmay be employedto tune one of more circuits to the same frequency andone or more circuits associated therewith to a different frequency overa wide range of fre quencies, thus eliminating the necessity forcondensers of special shape, such as those of the straight linefrequency type heretofore used, and an angularity adjustment betweeneach condenser or other mechanical expedient for setting up thefrequency difference.

In general, it may be stated that each circuit should have a variablecapacity which for any position of the control means or value of thevariable capacity in the other circuit, with which capacity it isunicontrolled, its value will be the same percentage of its total ormaximum operating value as permitted by movement limit of the controlmeans, as the value of the other capacity is to its total or maximumoperating value as permitted by movement limit of the control means.This means that the tuning capacities must have the same tuningcharacteristic, which is provided in the case of rotary plate condensersby having the plates of all of a unicontrolled series, of the sameshape.

The tuning capacities are preferably provided by identical condensers,or condensers having a differing number of plates but all of the sameshape, and with no angularity spacing or displacement between rotorswhen using the ordinary rotary type of variable condenser. Stated inanother way the tuning capacities not only have the sametuningcharacteristic as above mentioned, but are operated in step or insynchronism with each other. This preferred arrangement for such tunedcircuit for practical production is an inductance shunted by the tuningand series capacities in series, and with the series capacity-adjustableas by atrimmer or adjustable capacity in shunt therewith, and withtheoverall distributed capacity adjustable as by a trimmer or adjustablecapacity in shunt with the inductance or preferably with the tuningcapacity. 1

Previous mention was made of the fact that adjustment of theseriescondenser by such means as a compensating condenser will affect the lowfrequency end of the range considerably more than the high frequency endof the range, and likewise that adjustment of the shunt distributed orfixed ca-.

pacity either directly across the inductance or only across the variabletuning condenser has a predominant effect only at the high frequency endof the range. This variably tuned circuit has therefore a distinctadvantage over the customary simple circuit in that alignment ofcascade, circuits is great- 1y facilitated by these two adjustments,while the ordinary circuit without the series condenser can haveonly theone adjustment of distributed capacity which affects mainly the highfrequency alignment. It, therefore, follows that this new circuit.substantially as shown in Fig. 7, may be employed to advantage in radiofrequency amplifiers where it may be desired to have two compensatingadjustments on each resonant circuit to improve unicontrol alignment ofthe circuits.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. In an electrical apparatus, the combination with a pair of tunedradio frequency circuits each comprising an inductance and i a variabletuning capacity, said capacities having the same tuning characteristicand being connected for simultaneous'operation through the same tuningmovement range, of means for maintaining said circuits at substantiallya constant diiierence in frequency throughout said range, said meanscomprising a capacity connected in series with one of said tuningcapacities and having a value greater than the maximum value of saidtuning capacity, and a capacity connected in shunt to said tuningcapacity and having a value less than the minimum value of saidmaintaining said circuits at substantially a constant difference infrequency throughout said range, said means comprising a capacityconnected in series with one of said tunin capacities, and having avalue substantially three times the maximum value of said tuningcapacity, and a capacity connected in shunt'to said tuning capacity andhaving value less than the minimum value of said tuning capacity.

3. In an electrical apparatus, the combination with a plurality of tunedradio frequency circuits each comprising an inductance and variabletuning capacity connected therewith, said capacities having the sametuning characteristic and being connected for simultaneous operationthrough the same tuning movement range, of means for maintaining one ofsaid-tuned circuits at a substantially constant difference in frequencyfrom at least one of the other tuned circuits throughout the tuningrange of the variable capacities, said means comprising a fixed capacityconnected in series with the variable tuning capacity of one of saidcircuits, an adjustable capacity connected in shunt with said seriescapacity for aligning the circuit to a desired frequency diflerence withrespect to said other tuned circuits at the lower frequency end of thetuning range, said fixed and adjustable capacitieshaving a combinedcapacity. greater than the maximum operating value of' the variabletuning capacity, and means for adjusting the distributed. capacity ofsaid circuit for aligning the same to the desired frequency differencewith respect to said other tuned circuits at the higher frequency end ofthe tuning range.

4. In an electrical apparatus, the combination with a plurality of tunedradio fre quency circuits each comprising an inductance and a variabletuning capacity connected'therewith, said capacities having the sametuning characteristic and being con nected for simultaneous operationthrough the same tuning movement range, of means for maintaining one ofsaid tuned circuits at substantially a constant difierence in frequencyfrom at least one of theother tuned circuits throughout the tuning rangeof the variable'capacities, said means comprising a fixed capacityconnected in series with the variable tuning capacity of one of saidcircuits, an adjustable capacity connected in shunt with said'seriescapacity for aligning the circuit to a desired frequency difference withrespect to said other tuned circuits at,

the lower frequency end of the tuning range, said fixed and adjustablecapacities having a combined capacity greater than the maximum operatingvalue of the variable tuning capacity, and means for adjusting thecapacity of said circuit-for aligning the same to the desired frequencydifi'erence with respect to said other tuned circuits at the higherfrequency end of the tuning range, said means comprising an adjustablecapacity in shunt with the variable tuning capacit and having a maximumvalue less than t e minimum operating value of said tuning capacity.

5. In an electrical apparatus, the combination with a pair .of tunedradio frequency cir cuits, each comprisingan inductance and a variabletuning capacity, said capacities having the same tuning characteristicand bein connected for simultaneous operation throug the same tuningmovement range, of means for maintaining said circuits at substantiallya constant difference in, frequency throughout said range, said meanscomprising an adjustable capacity connected in series with one of saidtuning capacities and having a value greater than the maximum value ofsaid tuning capacity, and an adjustable capacity connected in shunt tosaid tuning capacity and having a value less than the minimum value ofsaid tuning capacity.

6. In an electrical apparatus, a plurality of tuned radio frequencycircuits having similar inductances and variablecapacities, means forvarying said capacities simultaneously, said capacities being so relatedthat for any position of said capacity varying means the value of onecapacity is the same percentage of its maximum value as the value of theother capacity is of its maximum value and the inductance and capacityof one of said circuits being so related to each other that said circuitis tuned to substantially a constant difference in frequency withrespect to another of said circuits throughout the movement range ofsaid capacity varying means.

cuit connected with one ofsaid condensers including inductance, shuntcapacit and series capacity values such that its requenc response periodis maintained at 'substantia 1y a constant difference with respect tothat of the other radio frequency circuit throughout said range.

8. ha tuned radio frequency a paratus, the combination ofa plurality ounicontrolled circuit-tuning variable condensers having the same tuningcharacteristic whereby they are operable with the same percentage ofmaximum change in capacity in each for a given unicontrolled movement, aseries capacity connected in circuit with one of said condensers andhaving a value greater than the maximum operating capacitygvalue of thecondenser with whic i and condenser including a distributed capacity ofa'value less than the minimumoperat including an inductance and a rotaryvariable tuning condenser connected therewith, of an oscillator circuit,a rotary variable tuning condenser for said oscillator circuit havingthe same plate shape and movement range as said first-named condenser,means for op-' erating said condensers in step simultaneously throughsaid movement range, said oscillator circuit including an inductance, acapacity in series therewith and a capacity in shunt therewith, saidseries and shunt capacities being so related to each other "and to thecapacity range of said tuning condenser that said circuit is tuned to afrequency .diflering from that of the tuned radio frequency circuit bysubstantially a constant frequency throughout the movement range of saidopcomprising a variable condenser for tuning said circuit, aninductance, a capacity greater than the maximum capacity of saidcondenser connected in series with said condenser, and a capacity lessthan the minimum capacity of the condenser connected in shunt with saidcondenser.

11. The combination in an electrical apparatus embodyingelectricdischarge devices, of an oscillator circuit connected'with one of saiddevices and comprising a variable condenser for tuning said circuit, aninductance, and adjustable capacity connected in series with thecondenser and having substantially three times the maximum capacity ofthe condenser, and an adjustable capacity less than the minimum capacityof the condenser connected in shunt with said condenser.

'12. In an electrical apparatus embodying electric discharge devices, anoscillator cir t is connected, and means connected in circuit with saidcapacity cuit connected with one of said devices andmagnetically withthe in oscillator circuit, and a common circuit concuit connected withone of said devices and including a variable condenser for tuning saidoscillator circuit, an inductance, and a capacity connected in serieswith said condenser and having a greater value than the maximum capacityof saidcondenser, said circuit having adistributed capacity less thanthe minimum capacity of said condenser, a tuned radio frequency circuitconnected with another of said electric discharge devices, a secondvariable condenser having the same tuning characteristic as said firstcondenser for tuning said tuned radio frequency circuit, andmeans-connecting said condensers for simultaneous operation in stepthrougli the same movement range.

13. In an electrical apparatus embodying electric discharge devices, anoscillator circomprising a variable condenser for tuning said oscillatorcircuit, an inductance, a capacity connected in series with thecondenser, and having a value greater than the maximum capacity of saidcondenser, the distributed capacity of said oscillator circuit beingless than the minimum capacity of said condenser, a tuned radiofrequency circuit connected with another of said electric dischargedevices, a 'second variable condenser having the same tuningcharacteristic as the fi rst for tuning said radio frequency circuit,means connecting said condensers for simultaneous operation in stepthrough the same movement range, an inductance connected in the cathodecircuit of the last-named electric discharge device, said inductancebein coupled electrodhctance of said nection for each of saidcondensers.

14. In an electrical apparatus embodying electric discharge devices, thecombination of a unicontrolled tuning means compr sing a plurality ofvariable condensers having the same tuning characteristic and-operablesiinultaneously through the same tuning movement range in synchronismwith eaclrother, a plurality of radio frequency circuits connected withsaid electric discharge devices and with certain of said condensersandhaving substantially the same inductance and distributed capacitywhereby they are tuned to substantially the same frequency throughoutthe movement range of the condensers, a

coupling coil connected with one of said circuits in a cathode lead ofone of said electric discharge devices and externally of the tunedcircuit of said device, an oscillator circuit connected with another ofsaid variable condensers and including an inductance electromagneticallycoupled with said coil, a capacity greater than the maximum operatingcapacity of the last-named variable condenser connected in seriestherewith in the oscillator circuit. and a capacity connected in theoscillator circuit and having a value less than the minimum operatingcapacity of said variable condenser, said capacities and inductancebeing so related that the oscillator circuit is tuned by simultaneousmovement of its variable condenser with those of the radio frequencycircuits to substantially a constant frequency difference throughoutthe-tuning range with respect to the frequency to which said radiofrequency circuits are tuned.

15. In an electrical apparatus embodying electric discharge devices, anoscillator circuit connected with one of said devices and including avariable condenser providing a timing capacity therein, an inductance,an adjustable capacity connected in series with said condenser, andbeing greater than the maxiir um capacity of the condenser, and anadjustable capacity in said circuit less than the minimum capacity ofthe condenser, a tuned radio frequency circuit connected with another ofsaid electric discharge devices, a second variable condenser having thesame tuning characteristic as the first for tuning said radio-frequencycircuit, and means connecting said condensers for simultaneous operationin step through the same movement range.

16. The combination with a variable condenser unit comprising aplurality of condensers having the same tuning characteristic andconnected for simultaneous operation through the same movement range insynchronism, of a high frequency circuit connected with each of saidcondensers, and means in at least one of said circuits including arelatively largeseries capacity and a relatively small shunt capacityfor changing the frequency response thereof by substantially a constantdifierence with respect to that of the remaining circuits throughout thetuning range of the condenser unit.

17. In an electrical apparatus, an oscillator circuit, said circuitcomprising a variable condenser providing a tuning capacity therein, an"inductance, a capacity in series with the condenser, said capacitybeing greater than the maximum capacity of the condenser, and adistributed capacity in said circuit less than the minimum capacity ofthe condenser, a tuned radio frequency circuit, a second variablecondenser having the same tuning characteristic as the first providing atuning capacity in said radio-frequency circuit, means connecting saidcondensers for simultaneous operation thereof in step through the samemovement range, a combined oscillator and frequency changer tube, andmeans for coupling saidtube with each of said circuits, said meanscomprising a pair of series-connected capacities, each connected withone of said tuned circuits and a grid connection for said tube betweensaid capacities.

18. In an electrical apparatus embodying denser providing a tuningcapacity therein,

an inductance, a capacity in series with the condenser, said capacitybeing greater than the maximum capacity of the condenser, and adistributed capacity in said circuit less than the minimum capacity ofthe condenser, a tuned radio frequency circuit, a second variablecondenser having-the same tuning. characteristic as the first providinga tuning capacity in said radio-frequency circuit, means connectin saidcondensers. for simultaneous operation 1n step through the same movementrange, a combined oscillator and frequency changer tube, and" means forcoupling the grid of said tube with each of said circuits,

said means comprising a pair of series-connected capacities, eachconnected with one of said tuned circuits and a grid connectlon for saidtube between said capacities, the opacity connected with the oscillatorcircuit being less in value than the capacity connected with theradio-frequency circuit and both capacities being greater in value thanthe grid to cathode capacity of the tube.

In witness whereof, I have hereunto set my hand this 2nd day of October,1928.

WENDELL L. CARLSON.

